1. Technical Field
This invention relates to cells for aluminium production by electrolysis of alumina dissolved in an electrolyte based on molten cryolite, particularly using the Hall-Héroult process. It relates more particularly to a device and a method for detecting anode effects.
2. State of the Art
Metal aluminium is produced industrially by fused bath electrolysis, namely electrolysis of alumina in solution in a molten cryolite bath called an electrolyte bath, according to the well-known Hall-Héroult process. The electrolyte bath is contained in pots called “electrolysis pots” comprising a steel shell that is lined with refractory and/or insulating materials on the inside, and a cathode assembly positioned at the bottom of the pot. Anodes are partially immersed in the electrolyte bath. The expression “electrolytic cell” normally denotes the assembly comprising an electrolysis pot and one or more anodes.
The electrolytic current that circulates in the electrolyte bath and the pad of liquid aluminium through anodes and cathode elements, produces aluminium reduction reactions and also maintains the electrolyte bath at a temperature of the order of 950° C. by the Joule effect. The electrolytic cell is regularly supplied with alumina so as to compensate for the consumption of alumina produced by electrolysis reactions.
One essential factor for achieving uniform operation of an aluminium production pot by electrolysis of alumina dissolved in a molten electrolyte bath based on cryolite is to maintain an appropriate content of dissolved alumina in this electrolyte and consequently to adapt quantities of alumina introduced into the bath to the consumption of alumina in the pot.
Excess alumina creates a risk of the bottom of the pot getting clogged with undissolved alumina deposits that could transform into hard plates that could electrically isolate part of the cathode. This phenomenon then causes the formation of very high horizontal electrical currents in the metal of the pots that interact with magnetic fields to stir the metal pad and cause instability at the bath-metal interface.
Conversely, a lack of alumina may in particular cause the appearance of the “anode effect”, in other words polarisation of an anode with a sudden increase in the voltage at the terminals of the cell and the release of large quantities of gaseous fluorides and carbon fluorides (CFx) that have a high capacity to absorb infrared rays encouraging the greenhouse effect.
Several regulation processes have been developed to control the alumina feed.
In industrial processes, it is known that an indirect evaluation of alumina contents can be used by monitoring an electrical parameter representative of the concentration of alumina in the said electrolyte. This parameter is usually the variation of the resistance R at the terminals of the pot powered at a voltage U, including a counter-electromotive force Ue for example evaluated at 1.65 Volts and through which a current I passes such that R=(U−Ue)/I. Typically, processes for regulation of the alumina content consist of modulating the alumina feed as a function of the value of R and its variation with time. Many patents have been made based on this basic principle, until very recently (for example see French application FR 2 749 858 corresponding to U.S. Pat. No. 6,033,550).
Therefore, these regulation processes provide a means of maintaining the alumina content in the bath within a narrow and small range and thus obtaining current efficiencies of the order of 95% with acid baths, by simultaneously and significantly reducing the quantity (or frequency) of anode effects on pots that are counted as the number of anode effects per pot and per day (AE/pot/day), called the “anode effect rate”. This rate is between 0.15 and 0.5 AE/pot/day for the most recent electrolytic cells (that use point feed systems).
The increasingly strict requirements in terms of the emission of greenhouse effect gases are encouraging aluminium producers to search for means of further reducing anode effect rates.
Therefore the applicant has searched for economic solutions to these difficulties that could be applied on an industrial scale.